Some male hamsters will continue to try to mate with an already pregnant female. It is best to keep the pregnant female in a cage by herself to prevent fighting and harm to the babies after they are born. This is also important because the female hamster can get pregnant right after having babies. This is very very hard on the momma hamster to try to nurse her babies while pregnant with another litter. For more detailed information about pregnant female hamsters check my hub on the topic. Good Luck .